The invention is generally related to the field of fabricating copper interconnects in semiconductor devices and more specifically to a pre-copper deposition wet surface treatment.
As the density of semiconductor devices increases, the demands on interconnect layers for connecting the semiconductor devices to each other also increases. Therefore, there is a desire to switch from the traditional aluminum metal interconnects to copper interconnects. Unfortunately, suitable copper etches for a semiconductor fabrication environment are not readily available. To overcome the copper etch problem, damascene processes have been developed.
In a conventional interconnect process, the aluminum (and any barrier metals) are deposited, patterned, and etched to form the interconnect lines. Then, an interlevel dielectric (ILD) is deposited and planarized. In a damascene process, the ILD is formed first. The ILD is then patterned and etched. The barrier material is then deposited over the structure with the copper being formed over the barrier material. Then, the copper and barrier material are chemically-mechanically polished to remove the material from over the ILD, leaving metal interconnect lines. A metal etch is thereby avoided.
The most practical technique for forming copper interconnects is electrochemical deposition (ECD). In this process, after the barrier material is deposited, a seed layer of copper is deposited. Then, ECD is used to deposit copper over the seed layer. ECD is a process used to produce a solid phase product (such as thin films) by electrochemical reaction. Cu ECD is a process used to make Cu thin films through electrochemical reduction of Cu ion, represented by the following chemical equation:
Cu+++2exe2x88x92xe2x86x92Cu where exe2x88x92 represents electron.
In order for the ECD process to proceed, a seed layer is required to pass current and to serve as a nucleation layer. The wettability of the seed surface is very critical for the ECD process. If a seed surface cannot wet the plating solution, no Cu can be deposited on that surface area, forming a void. Proper control of the bath composition allows the ECD process to be carried out in a bottom-up mode (i.e., film preferentially grows from the bottom of small features to avoid seams or fill voids). This feature makes ECD preferable over other copper fill processes.
One of the problems with ECD copper films is that the films frequently show defects of deep voids. The deep voids have characteristic swirl marks across a wafer as shown in FIG. 1. This swirl defect is known to appear and disappear in an uncontrolled pattern. This causes wide lot-to-lot variations. Accordingly, a better understanding of the factors contributing to the ECD defects and a solution for reducing or eliminating the defects are desired.
The invention is a pre-ECD wet surface treatment. After forming the barrier material and seed layer, the surface of the seed layer is treated with a water-based solution to improve surface wettability. The ECD copper film is then formed over the seed layer.
An advantage of the invention is providing a stable manufacturing process for ECD of copper films.